Flowmeters are utilized in many different industries to measure and control the flow of various fluids. Flowmeters generally utilize moveable float members in the fluid flow stream for the measurement of pressure drops across an orifice in the fluid flow stream. These flowmeters generally have electrical circuits and readouts that provide highly accurate measurements of flow rates. Due to their complexity, reliability and maintenance are issues, as is cost. A mechanically simple and highly reliable flowmeter utilizes an upright tube that allows for visual gauging of volumetric flow rates through the monitoring of marked indicia on the sight flow tube itself, or other connection means. The sight tube will have a pair of fittings at each end of the sight tube for connection to and insertion into a fluid flow circuit. A “float” is denser than the fluid being measured, is visible through the sight tube, and rises up the tube as the flow rate increases. The flow rate is visually indicated by the position of the float in the sight tube. Typical floats are generally shaped as balls, spherical objects, and other non-elongate members designed to move freely in the sight tube or to be guided along a guide rod securely mounted within the sight tube. Such conventional float designs generally function sufficiently in measuring medium to high fluid flow rates through a flowmeter. However, in certain industries, such as semi-conductor processing, low and ultra-low fluid flow rates are often required during processing. The measurement of these reduced flow rates through a fluid flowmeter must be accurately indicated to ensure processing efficiency and precision.
Even known float assemblies in the industry having a generally elongate float, which are designed to meter low fluid flow rates, are deficient. Referring to FIG. 2 in particular, a prior art flowmeter 210 having a tapered elongate float 217 and sight tube 212 system is utilized wherein the float 217 is guided through guides 214, 216. This system is intended to meter low fluid flow rates. The float 217 comprises a tapered section 218 that ends approximately central to the float 217 at a ledge 222. Lateral float movement is controlled with the use of bottom guides 216 and top guides 214. The taper of the float 217 increases from one end proximate the guides 216 to the ledge 222. As the float 217 is forceably moved upward with fluid pressure through the sight tube 212, it progresses upward until the ledge 222 engages the top guides 214. With a reduction in fluid flow, the float 217 returns downward until being stopped by the tapering effect of the tapered section 218. Such a system has an innate drawback in that stopping of the float 217 with the tapered section 218 within the bore or channel of guides 216 can cause an undesirable wedging effect. This innate characteristic is particularly unacceptable when measuring low flow rates. Namely, the tapered section 218 can become measurably stuck within the guides 216 such that a higher level of flow is required to initiate forceable movement of the float 217 within the tube 212. Since low flow rates are the focus of such a flowmeter, this can serve to decrease reliability and accuracy, especially for the periods of fluid flow prior to dislodging of the wedged float 217. In fact, this may completely prevent fluid flow metering for ultra-low fluid flows through the flowmeter 210.
In the processing of semi-conductor wafers into integrated circuits, highly corrosive, ultra-pure fluids, such as hydrochloric, sulfuric and hydrofluoric acid, are in extreme temperature ranges and are utilized. These fluids not only damage traditional flowmeter materials, but they additionally impose significant health risks for personnel exposed to the fluids during the manufacturing process. Moreover, the equipment and materials in contact with these ultra-pure fluids must not contaminate or add impurities to the fluids.
Thus, semi-conductor processing applications require flowmeter construction providing accurate fluid flow measurements at varying fluid flow rates, while at the same time utilizing highly inert materials that withstand the potential damaging effects of these corrosive fluids, that do not contaminate the fluids, and that tolerate the broad temperature ranges. Moreover, the design of such flowmeters must minimize fluid leakage pathways.
Prior art flowmeters have addressed the problems associated with the use of corrosive fluids in flowmeters by using highly inert corrosive-resistant plastics in the construction of components of the flowmeters. Fluoropolymers such as perfluoroalkoxy resins (PFA), polytetrafluoroethylenes (PTFE), and ethylenetetrafluoroethylenes (ETFE) are plastics that are suitable for use with these corrosive fluids. The translucent-transparency characteristics of thin-walled PFA is typically utilized in the construction of the sight tube of these flowmeters.
U.S. Pat. No. 5,672,832 (the '832 patent) is an example of a flowmeter device where fluoropolymers are utilized. This specific device discloses a fluoropolymer housing flowmeter that places two cavities in the flow tube region where pressure sensors are placed for accurately measuring fluid flow rates. The rectangular housing and cover for this invention are constructed of non-translucent PTFE and the cover is mounted to the housing with screws, with a gasket positioned in between the two in an attempt to minimize fluid leakage.
U.S. Pat. Nos. 5,078,004, 5,381,826, and 5,549,277 are examples of fluoropolymer flowmeters utilizing sight tubes where a limited portion of the flowmeter is made of PFA material. In such flowmeters, the centrally located sight tube can be machined from PFA, with additional fitting components machined from PTFE, or other non-translucent materials, which are connected directly to the ends of the sight tube, or connected in series with those parts that do have a direct association with the PFA sight tube. Generally, each of these components are attached to each other and/or the sight tube via threaded portions.
These currently available fluoropolymer flowmeter devices, whether they be conventional sight tube flowmeters or other flowmeters, contain disadvantages centering mainly around the materials used and the methods of assembly.
Generally fluoropolymers, particularly PTFE, are not conducive to injection molding processes. As a result, in the known commercial sight tube fluoropolymer flowmeters, such as the device shown in FIG. 1, each component is machined to obtain the desired shapes, tolerances, and the requisite threaded connections. Machining adds very significant labor costs to the production of the devices and, to the extent possible, should be avoided. Moreover, multi-component flowmeter assemblies utilizing threaded portions present potential fluid leakage pathways. The possibility of fluid leakage is increased with each non-unitary connection between components. For instance, in FIG. 1, the flowmeter 200 includes at least a first fitting 202, and a second fitting 204 that are threadably attached, at threaded portions 208, to the tapered sight tube 206, thus increasing the potential for unacceptable leakage. Further, the sight tube 206 is likely constructed of translucent PFA, while the fittings 202, 204 are constructed of a material such as PTFE.
Ideally, flowmeters, particularly those utilized in handling corrosive-caustic fluids, should have a minimum number of non-unitary connections that do utilize the process of threadingly joining molded flowmeter components, namely the fittings to the sight tube.
The manufacturing process for the so-called unitary-bodied flowmeters constructed of conventional plastics generally involves the affixation of a plug or cap to a body portion. The affixation processes known for these conventional plastic sight tube flowmeters involve adhesive bonding and ultrasonic welding. Ultrasonic welding involves vibrating or oscillating a first plastic component with respect to a second plastic component that it is in engagement with the first plastic component. Such welding is not effective for joining tubular end portions. Moreover, due to the “slippery” nature of fluoropolymers, forms of vibrating or oscillating bonding is not realistic. Similarly, adhesives do not work on fluoropolymers, and would only add potential contaminants which must be avoided in semi-conductor processing applications.
Although PFA is substantially more expensive then PTFE (perhaps 10–15 times as expensive) it is considered to have great advantages over PTFE. Namely, PFA is cleaner, providing less contaminants than PTFE. Further, and unlike PTFE, PFA can be injection molded and homogeneously joined with like materials.
Homogeneously joining by welding separate fluoropolymers components, such as PTFE, is essentially impossible. In comparison, PFA components may be welded together utilizing non-contact heating as disclosed in U.S. Pat. No. 4,929,293, assigned to Fluoroware, Inc., also the owner of the instant application. It is believed that these welding techniques have never, before this invention, been utilized in association with the manufacture of a fluoropolymer flowmeter.
All of the discussed prior art falls short of adequately addressing the unique accuracy, purity, and low fluid flow needs of the semi-conductor processing industry. The prior art does not address the need for coupling the benefits PFA offers in resisting corrosion with the advantages a unitary-bodied component construction advances with regard to leakage prevention and reduced manufacturing and assembly costs.